Nanomaterials refer to those with at least one dimension in three-dimensional space within the nanoscale range (1-100 nm) or materials constituted by them as basic units. Nano-size effects often exhibit melting points and magnetic, optical, thermal conduction and electrical conduction characteristics which are different from bulk materials, so they can be widely used in optoelectronic materials, ceramic materials, sensors, semiconductor materials, catalytic materials, medical treatment and other fields.
Currently, the methods for synthesis of nanomaterials mainly include three categories: solid phase method, liquid phase method and gas phase method. Since nanomaterials have high surface energy, it is hard for nanomaterials obtained by various solid-phase synthesis methods, including high-temperature calcination and mechanical milling, to reach the characteristics of ultrafine size, narrow size distribution and high dispersibility. Gas phase method is an important method for synthesis of ultrafine nanopowder, for example, vapor deposition method is the technology most widely used in semiconductor industry to deposit a variety of nanomaterials, but this process has higher equipment requirements, and deposition of nanoparticles in matrix is often accompanied with permanent agglomeration of the nanoparticles, so it is difficult to ensure their monodispersity. Compared to the former two methods, liquid phase method can prepare nanomaterials of various morphologies and sizes by selecting appropriate solvents and additives. Currently, due to low cost and wide sources, water is the most commonly used solvent in liquid phase method. However, water has high polarity, the raw materials for synthesis of nanomaterials react fast in this medium, and it is very difficult to simply use water as the solvent for controlled synthesis of ideal nanomaterials, so improvement is generally made by adding surfactants and changing synthesis process. In almost all the methods currently reported in literatures and patents for synthesis of nanomaterials of monodispersity, small size and narrow distribution with water solvent system, surfactants or surfactant-like additives are used. These surfactants or additives not only increase the cost for preparation of nanomaterials, but also inevitably remain on the surface of nanomaterials, thus affecting their subsequent use, for example, residues may intoxicate some catalytic nanomaterials, affect biocompatibility of medical nanomaterials, etc.
Recently, a literature has reported use of organics as the solvents for synthesis of nanomaterials, such as the “alcohol-thermal method” using ethanol, ethylene glycol, propylene glycol, polyethylene glycol, etc. as the solvents and high-temperature pyrolysis, with oleic acid as the solvent, of metal oleate, carbonyl salts or acetylacetone to prepare oxides, but these methods still require to use surfactants or expensive organic metal salts, and even some synthetic routes need to consume organic solvents to provide oxygen required to produce oxides, which is not conducive to application of these methods [see specific literature: Magnetite Nanocrystals: Nonaqueous Synthesis, Characterization, and Solubility, Chem. Mater 2005, 17, 3044-3049].
Lactam is an organic compound containing amide groups in cycles, for its derivatives, some nitrogen atoms and hydrogen on carbon can be substituted by other groups, among which, butyrolactam (also called α-pyrrolidone) containing four carbon atoms is liquid at room temperature and a commonly used high-boiling-point polar solvent in organic systhesis. Recently, Gao, et al, has synthesized super paramagnetic ferroferric oxide <20 nm in size with carbonyl iron and ferric trichloride in butyrolactam solvent [see specific literature: One-Pot Reaction to Synthesize Water-Soluble Magnetite Nanocrystals, Chem. Mater., Vol. 16, No. 8, 2004; Preparation of Water-Soluble Magnetite Nanocrystals from Hydrated Ferric Salts in 2-Pyrrolidone: Mechanism Leading to Fe3O4, Angew. Chem. Int. Ed. 2005, 44, 123-126]. Butyrolactam used in this method itself is a highly toxic solvent, which brings great threat to production safety and environmental protection. More importantly, butyrolactam has a strong coordination effect and can be firmly adsorbed on the surface of nanomaterials, which will certainly affect the subsequent use of nanomaterials containing toxic components, especially in medical treatment, health, food and other fields. In addition, other literatures were restricted to synthesis of nano ferroferric oxide with butyrolactam as the solvent, but not investigated other oxides, hydroxides and metal nanomaterials.
Among lactams, except butyrolactam, cyclic lactams containing 5 or more carbon atoms in cycles are solid at room temperature, and the melting point increases with the number of carbon atoms, for example, the melting point was 39° C. for valerolactam, 68° C. for caprolactam and up to 153° C. for laurolactam, so it is difficult to think about through butyrolactam that lactams containing 5 or more carbon atoms should be used as the solvents for organic synthesis, and moreover, there is no report on use of them alone as the solvents for synthesis of nanomaterials. Such substances have the structures similar to that of butyrolactam, and at temperature higher than their melting points, they have relatively strong polarity, but weaker than the polarity of water, so they can not only guarantee certain solubility of raw materials for synthesis of nanomaterials in the solvents, but also slow down the reactions, thus they are a kind of ideal solvents for synthesis of nanomaterials. Most importantly, lactam's amide groups have a coordination effect and can play a role similar to surfactant, so no other surfactants are required for synthesis of nanomaterials when using this solvent. Furthermore, lactam derivatives contains two amide groups in the cycles, for example, succinimide, glutarimide, adipimide also have similar characteristics. Therefore, lactams containing 5 or more carbon atoms, above their melting points, can substitute other common solvents for preparation of nanomaterials.